The objective of this investigation was to determine the effects of restricted movement and forced exercise on protein metabolism and body composition of adult rats. Half of the animals were restricted in movement and half of each group were forced to exercise by swimming for ten days. The study entailed a six day adjustment period, a four day balance study, and a ten day exercise period followed by another four day balance study. Weight change, food consumption, nitrogen retention, liver composition, and body composition were analyzed to test the effects of the treatments. Restricted animals retained less nitrogen than the controls, but negative nitrogen retentions expected when animals were losing weight did not occur. Changes in body fat correlated positively with body weight change. After about fifteen days of restriction, the animals seemed to adapt to the inactivity. This was indicated by an increase in food consumption and a dramatic change from weight loss to weight gain. Considerable variation existed in the ability of individual animals to adapt. Some animals did not adapt at all, and died early in the study. Other animals adapted readily and gained weight in excess of their initial weight loss. Ten minutes of swimming daily for ten days did not cause observable effects on the restriction. The adaptation seems to indicate that the expected weight loss and decreased nitrogen retentions will not be a problem. / M.S.